Conventionally, polystyrene type resin foamed molded products obtained by filling a mold with polystyrene type resin prefoamed particles and then heating to effect foaming are known to exhibit excellent rigidity, thermal insulation properties, lightweight properties, water resistance and foam moldability. As a result, these foamed molded products are used widely as cushioning materials and thermal insulation materials for construction materials. However, these foamed molded products suffer from poor levels of chemical resistance and impact resistance.
On the other hand, foamed molded products formed from a polypropylene type resin are known to exhibit excellent chemical resistance and impact resistance. As a result, these foamed molded products are used in automotive components.
However, because polypropylene type resins exhibit poor retention of the foaming gas, the foam molding conditions must be controlled very precisely, which leads to a problem of increased production costs. In addition, polypropylene type resin foamed molded products exhibit inferior rigidity to polystyrene type resin foamed molded products.
In order to address these problems associated with polystyrene type resins and polypropylene type resins, foamed molded products obtained by complexing a polystyrene type resin having favorable rigidity and foam molding properties with a polypropylene type resin having favorable chemical resistance and impact resistance have been proposed (for example, see Patent Documents 1 and 2).
Patent Document 1 has an object of providing a foamed molded product with practically applicable levels of impact resistance, rigidity and surface properties and the like, and discloses a method of obtaining prefoamed particles formed from a modified polypropylene type resin composition. This modified polypropylene type resin composition is obtained by heating an aqueous suspension containing 100 parts by weight of a polypropylene (a), 1 to 500 parts by weight of a vinyl type monomer component (b), and 0.01 to 10 parts by weight of a radical polymerization initiator per 100 parts by weight of the component (b), in some cases under conditions in which the vinyl type monomer component (b) undergoes no substantial homopolymerization; subsequently impregnating the polypropylene (a) with the vinyl type monomer component (b); and then further heating the aqueous suspension to at least a temperature at which the crystalline portion of the polypropylene (a) begins to melt substantially, thereby polymerizing the vinyl type monomer component (b).
Patent Document 2 discloses a method of obtaining spherical polypropylene type resin particles, the method including: dispersing pellets or a powder of a propylene type resin containing from 1 to 10% by weight of an ethylene component in an aqueous suspension, adding approximately 30 to 150 parts by weight of a styrene type monomer to the suspension and heating at a temperature of 130° C. or higher, and then conducting a polymerization in the presence of a polymerization catalyst.
However, in the modified resins obtained in the conventional methods disclosed in Patent Documents 1 and 2, dispersing the polystyrene type resin in a particulate form within the polypropylene type resin is difficult, particularly in the regions near the surface of the resin, meaning the resin often exhibits unsatisfactory chemical resistance.
Further, molded products produced by foam molding using modified resin particles produced using these conventional methods are unable to satisfactorily combine heat resistance with penetration impact resistance.
[Patent Document 1]
Japanese Unexamined Patent Application, First Publication No. Hei 9-194623
[Patent Document 2]
Japanese Unexamined Patent Application, First Publication No. Sho 61-9432